First, in accordance with finely structural and integrated forming of a semiconductor device, management of lithography steps has been more and more increased a request for measuring a fine pattern of a size of several tens nm formed on a wafer with high accuracy and at high speed, and a Critical Dimension Scanning electron Microscope (hereinafter, CD-SEM) is a measurement device which is indispensable for an accuracy of a semiconductor. In recent years, a need for measuring a two-dimensional pattern has also been increased in addition to a need for measuring a line width of a standard line and space (L&S) pattern. The measurement of the two-dimensional pattern is realized by comparing an SEM image with LSI layout data of a format of GDSII or the like which is a de facto standard for describing a mask pattern of a semiconductor. In the management of lithography steps, enormous LSI layout data, observation points which are necessary for measuring a two-dimensional pattern may also be as many as several tens thousands points/chip, and there are also high needs for shortening time periods of move/image acquire/measure: Move Acquire Measure (hereinafter, MAM). Design For Manufacturing Scanning electron Microscope (hereinafter, DFM-SEM) which can meet the needs described above is needed.
Japanese Unexamined Patent Application Publication No. 2006-196281 (Patent Literature 1) discloses a method of holding an adjusted value of a gain of a detector or a detection algorithm by setting plural beam currents and switching the beam currents at high speed in order to make S/N and shortening of an image taking time period compatible with each other by switching the beam currents at a scanning electron microscope.
Japanese Unexamined Patent Application Publication No. Hei3-229179 (Patent Literature 2) discloses a method of improving S/N of a signal by calculating a sum of sampling values at plural points of pulses of secondary electron signals.
Japanese Unexamined Patent Application Publication No. 2006-105977 (Patent Literature 3) discloses an imaging system including a detector adjusting circuit which determines an operation parameter of a detector on which a radiation is incident from a radiation source based on the incident radiation.
Second, a semiconductor device is fabricated by repeating steps of transcribing a pattern formed on a wafer by a photomask by a lithography processing and an etching processing. In such a fabrication process, it is indispensable to swiftly analyze a defect discovered by an in-line wafer inspection and utilize the analysis for the countermeasure in order to realize an increase in a yield at an early stage and stable operation of a fabrication process. A technology of reviewing a large number of detection defects and classifying the detection defects according to causes of defect occurrence is needed to link an inspection result swiftly to a countermeasure against a failure.
However, a defect size which effects an influence on a fabrication yield of a semiconductor is made to be fine in accordance with fine structure formation of a fabrication process. According to an optical reviewing device of a background art, it is difficult to review and classify a small defect owing to a deficiency in a resolution. A review device of an SEM (Scanning Electron Microscope) type capable of reviewing a small defect with high resolution is therefore used. According to the device, it is important to acquire a shadow image by an SEM image that is equivalent with a shadow image produced when a light beam is cast sideways in order to detect recesses and protrusions of a small foreign matter, a scratch or the like.
An explanation will be given of a general principle for acquiring such a shadow image in reference to FIG. 1. For example, when a recessed and protruded portion 1 at a sample surface that is caused by, for example, a foreign matter in a film is scanned by an electron beam 2, secondary particles (secondary electrons) 3 are emitted at respective irradiation points on the sample. Here, an energy of the secondary electron 3 emitted has a distribution, a component having a comparatively low energy (low speed component) is referred to as a secondary electron (SE), and a component having a comparatively high energy (high speed component) is referred to as a back scattering electron (BSE). Secondary particles at a generated portion have elevation angle components in various directions as indicated by numeral 6 of FIG. 1 by an arrow mark. Here, the elevation angle of the secondary particle at the generated portion signifies an angle which is made by each elevation angle component of the secondary particle relative to a plane at which an optical axis of the irradiated primary beam becomes a normal line. When attention is paid to a certain one of the elevation angle component 6 of the secondary particle at the generated portion, although the second particle which is emitted to the right side reaches a detector 4, the secondary particle which is emitted to the left side does not reach the detector. An amount of detecting a secondary electron at the detector 4 therefore differs by an inclination angle 5 of the recessed and protruded portion of the sample surface at the generated portion of the secondary particle. As the result, a shadow contrast in accordance with the recesses and protrusions of the sample surface appears at a shadow image 7 that is acquired by the detector.
Japanese Unexamined Patent Application Publication No. Hei8-273569 (Patent Literature 4) discloses a technology with regard to a charged particle beam column which improves a measurement accuracy of a sample by separating to detect a low speed component (SE) and a high speed component (BSE) of a secondary particle in an optical system of detecting a secondary charged particle using an electromagnetic superposing type objective lens. According to the technology disclosed in the publication, the low speed component and the high speed component are separated to detect by detecting BSE at an inner side ring-like band and SE at an outer side ring-like band by a ring-like detector provided between an electron source and an objective lens by utilizing the fact that trajectories of the low speed component and the high speed component of the secondary particle differ from each other. Since the outer side ring-like band is divided into four in a fan shape and an azimuthal angle of a secondary electron can be selected at an emitting position, a shadow image can be acquired.
On the other hand, International Publication No. WO00/19482 pamphlet (Patent Literature 5) discloses a configuration for separating to detect a low angle component and a high angle component of a secondary particle. According to the configuration disclosed in the publication, a secondary particle detector for detecting a low angle component on an upper side of an objective lens is provided, a reflecting plate for impinging the low angle component of the generated secondary particle between the detector for detecting the low angle component and the objective lens is arranged, a subsidiary particle generated by impinging the low angle component particle is guided to a secondary particle detector for detecting the low angle component by an E×B deflector, and accordingly, the low angle component of a reflected electron and a secondary electron are detected. With regard to a high angle component of the reflected electron, a separate secondary particle detector detecting the high angle component and a second E×B deflector are provided on an upper stage (electron source side) of the E×B deflector to detect only the high angle component by the detector for the high angle component.
Japanese Unexamined Patent Application Publication No. 2006-228999 (Patent Literature 6) discloses an electron microscope which is provided with a ring-like detector between an electron source and an objective lens for selecting to detect a low angle component and a high angle component of an elevation angle, and an azimuthal angle component of a secondary electron generated.